Fluidized bed pyrocarbon coating

ABSTRACT

Arrangements are provided for assembling multiple substrates for coating within a fluidized bed coater so as to deposit a coating of uniform thickness across the entire exterior surface thereof. One embodiment includes a method for coating orthopedic implants having convex and concave surfaces with pyrocarbon by pyrolytic decomposition of a hydrocarbon.

This application is a continuation of U.S. patent application Ser. No.13/032,213, filed Feb. 22, 2011, which claims priority to United StatesProvisional Patent Application No. 61/306,819 filed on Feb. 22, 2010entitled “Fluidized Bed Pyrocarbon Coating.” The content of each of theabove applications is hereby incorporated by reference.

BACKGROUND

Pyrolytic carbon, which is referred to herein as pyrocarbon, isgenerally deposited by thermally decomposing a gaseous hydrocarbon (orother carbonaceous substance) in vapor form as a coating upon relativelysmall substrates that can be levitated in a fluidized bed along with anancillary charge of small particles. In the time that has passed sincethe development of fluidized bed technology for applying pyrolyticcarbon coatings, as exemplified for example in U.S. Pat. No. 3,977,896to Bokros et al. entitled “Process for Depositing Pyrocarbon Coatings”,it has been discovered that there are many variables with respect to thefluidized bed environment that may determine the structure of thepyrocarbon that is deposited. Because pyrocarbon (having amicrostructure that is free of growth features) is generally depositedwhen the relative amount of deposition surface area to volume in afluidized bed is fairly high, such small objects to be coated withpyrocarbon may be tumbled in a bed of minute particles. In many cases,the substrates being coated, which are larger than the minute particles,will exhibit substantially random motion within the fluidized bed. As aresult, the surface area of the substrate will generally besubstantially equally exposed to the upward flow of the mixture ofhydrocarbon and inert gas flowing through the coating enclosure.Consequently all or most surfaces will receive a substantially uniformthickness of pyrocarbon. However, when such relative uniformity oftumbling motion does not occur, possibly because of the particulargeometry of the substrates being coated, uniformity of coating thicknessmay not be achieved.

Specifically, the configurations of some objects or substrates have atendency to assume a relatively stable orientation in an upwardlyflowing fluidized bed despite collisions with other substrates and theparticles in the bed. This tendency is referred to as a tendency to“plum-bob” (i.e., to float in one particular orientation within the bed,and not randomly tumble). When such plum-bobbing occurs, an inadequatecoating thickness may be applied to surface regions that are essentiallyshielded or hidden from direct contact with the upwardly movinghydrocarbon stream and “free carbon”. This may be because the shieldedsurface regions experience pyrolysis and deposition of pyrocarbon at aslower rate. The result of such a situation can be an unacceptably thindeposition of pyrocarbon coating in these regions and therefore overallnon-uniformity of pyrocarbon thickness across the entire surface of thesubstrate. For instance, a thin hemispherical shell with a stemprotruding axially from within the concave surface (e.g., an orthopedicimplant) may float within the coating bed with the convex face facingdownward receiving a desired amount of coating while the base of thestem (where it affixes to the concave face of the shell) ispredominately shielded from the coating and receives an unacceptablythin deposition of carbon.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the present invention willbecome apparent from the appended claims, the following detaileddescription of one or more example embodiments, and the correspondingfigures, in which:

FIG. 1 is a front view of an assemblage of six substrates to be coatedin a fluidized bed coating operation confined in spatial relation to oneanother by a surrounding holder.

FIG. 2 is a perspective view of an assemblage of substrates, the same asthose shown in FIG. 1, which are confined by a holder of slightlydifferent shape.

FIG. 3 is a view similar to FIG. 1, showing an assemblage of six thinhemispherical shell substrates, which are held in regular spatialorientation one to another by a holder of a different construction.

FIG. 4 is a perspective view of the assemblage shown in FIG. 3.

FIG. 5 is a schematic view, partially in section, of a prior artfluidized bed coater.

FIGS. 6A and 6B are side and perspective views of an embodiment of theinvention.

FIGS. 7A-E are side views of components of the embodiments of FIGS. 6Aand 6B.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. Well-known structures andtechniques have not been shown in detail to avoid obscuring anunderstanding of this description. References to “one embodiment”, “anembodiment”, “example embodiment”, “various embodiments” and the likeindicate the embodiment(s) so described may include particular features,structures, or characteristics, but not every embodiment necessarilyincludes the particular features, structures, or characteristics.Further, some embodiments may have some, all, or none of the featuresdescribed for other embodiments. Also, as used herein “first”, “second”,“third” describe a common object and indicate that different instancesof like objects are being referred to. Such adjectives are not intendedto imply the objects so described must be in a given sequence, eithertemporally, spatially, in ranking, or in any other manner. Also, theterms “coupled” and “connected,” along with their derivatives, may beused. In particular embodiments, “connected” may be used to indicatethat two or more elements are in direct physical or electrical contactwith each other and “coupled” may mean that two or more elementsco-operate or interact with each other, but they may or may not be indirect physical or electrical contact. Also, while similar or samenumbers may be used to designate same or similar parts in differentfigures, doing so does not mean all figures including similar or samenumbers constitute a single or same embodiment.

When parts or substrates are coated in a fluidized particle bed whereindecomposition of a vapor occurs, such coating is carried out in a heatedenclosure through which there is an upward flow of gas (e.g., a streamof a mixture of an inert or diluent gas and the feed gas or vapor thatis being decomposed). Gas flow rates are controlled so as to levitatethe bed of minute particles which generally alternatively rise and fallin a somewhat toroidal flow path. The substrates being coated are of asize and shape such that they are levitated by the upward flowing gasstream and thus are essentially suspended within the bed wherein theyrandomly tumble. The bed is heated to a temperature at which the feedgas decomposes and deposits a thin film coating upon the substrates andthe particles. A small fraction of the particles are usually regularlyremoved and replaced so as to maintain bed surface area at a generallyconstant level. When the substrate shapes are such that they tumblerandomly within the fluidized bed, all of the surfaces thereof arefairly uniformly exposed to the flowing stream of gas, and as a result,have a substantially uniform thickness coating deposited thereupon.Because substrates of certain size (e.g., uneven weight distributionresulting in a “heavy” side and a “light” side) and shape (e.g.,asymmetric) have a tendency to float within the bed in a substantiallysingular orientation (e.g., plum-bob or do not randomly tumble), thencertain surfaces of such substrates are exposed directly to the upwardflowing gas stream more than others, and the coating thickness depositedis no longer uniform.

However, by grouping multiple substrates to be coated by vapordeposition together as an assemblage, with the individual substratesdisposed at regular spherical orientation one to another, the assemblagewill be levitated within the coater with a regular tumbling movement. Asa result, all of the surfaces of the substrates will randomly tumble andbe exposed to about the same extent to the upwardly flowing gas stream,and the thickness of the coating deposited will be substantially uniformover the entire exposed surfaces of the substrates.

In an embodiment, non-uniform thickness coatings can be overcome byassociating groups of such similar substrates and confining them so asto have a substantially fixed orientation spatially one to another;then, when the confined group is levitated in an upwardly flowing gasstream, high uniformity of coating thickness can be achieved. Excellentresults are obtained when coating with pyrocarbon in a bed of fluidizedparticles.

An embodiment includes a method for coating multiple substrates in afluidized bed of particles by depositing pyrocarbon thereupon in amanner to obtain uniform coating thickness across each coated substratesurface, which method comprises the steps of establishing a bed ofparticles in a fluidized condition in a coating zone within anenclosure, which particles are levitated by supplying an upward flow ofa gaseous atmosphere (which may comprise an inert gas and/or ahydrocarbon and/or other gases), providing a group of substrates ofgenerally similar shape and size to be coated, confining the group ofsubstrates in association with each other so as to have a substantiallyfixed orientation spatially one to another, levitating the confinedgroup of substrates within the fluidized particle bed so that theassociation of substrates tumbles randomly within the bed, and heatingthe fluidized bed to a temperature such that the hydrocarbon decomposesto deposit pyrocarbon, whereby surfaces of the substrates upon whichpyrocarbon is being deposited are uniformly exposed to upward gas flowand, as a result, a coating of pyrocarbon of substantially uniformthickness is deposited upon the substrates throughout the surface areathereof.

An embodiment includes a method for coating multiple substrates in afluidized bed of particles by vapor decomposition to deposit thereuponin a manner to obtain uniform thickness of coating, which methodcomprises the steps of establishing a bed of particles in a fluidizedcondition in a coating zone within an enclosure, which particles arelevitated by supplying an upward flow of a gaseous atmosphere comprisinggas (e.g., inert gas and a thermally decomposable vapor), providing agroup of substrates of generally similar shape and size to be coated,confining the group of substrates in association with each other so asto have a substantially fixed orientation spatially one to another,levitating the confined group of substrates within the fluidizedparticle bed so that the association of substrates tumbles randomlywithin the bed, and heating the fluidized bed to a temperature such thatthe vapor decomposes to deposit a coating upon the substrate, wherebysurfaces of the substrates are uniformly exposed to upward gas flow and,as a result, a coating of pyrocarbon of substantially uniform thicknessis deposited upon the substrates throughout the surface area thereof.

An embodiment provides an assemblage for coating multiple substrates inan upwardly flowing gaseous stream so as to deposit a coating of uniformthickness across the coated surfaces, which assemblage comprises a groupof substrates of generally similar shape and size to be coated, and acontainer or cage confining the group of substrates in association witheach other so as to have a substantially fixed orientation spatially oneto another, whereby the confined group of substrates can be levitatedwithin a coater to which an upward flow of gaseous atmosphere issupplied wherein the assemblage will tumble randomly, as a result ofwhich surfaces of the substrates are uniformly exposed to upward gasflow and have a coating of substantially uniform thickness depositedthereupon.

The operation of various embodiments is described herein with respect toa pyrocarbon coating process where a gaseous hydrocarbon ispyrolytically decomposed in a fluidized bed of minute particles.However, it should be understood that embodiments might beadvantageously practiced with other vapor decomposition coating methods(e.g., methods where there is levitation of the substrates within anupwardly flowing gas stream containing a feed gas or vapor that will bechemically decomposed by the application of heat to deposit a coating onthe surfaces of the levitated substrates). When pyrocarbon is thedesired product, the coater may be a fluidized bed coater and upwardlyflowing gas stream may be a mixture of feed gas in the form of propaneor propylene, for example, and an inert gas such as argon, nitrogen orhelium.

Illustrated in FIG. 5 is a conventional fluidized bed coating apparatus20 which includes a furnace 22 having an outer cylindrical shell 24within which a coating enclosure is located. Details of the constructionand operation of the fluidized bed coating apparatus are set forth inU.S. Pat. No. 5,284,676, the disclosure of which is incorporated hereinby reference. The furnace 22 is supported upon a water-cooled supportplate 25 to which it may be bolted. A coating enclosure is generallyseparated from the outer cylindrical shell 24 by a layer of insulation24 a and is defined by an elongated sleeve 26 which fits about the upperend portion of a lower nozzle block 28 that has an interior conicalbottom surface 30 which extends upward from a vertical centralpassageway 32 that is coaxial with the exterior of the nozzle block andthe elongated tube 26, both of which are circular in cross-section. Theupper end of the furnace includes an annular spacer 33, which centersthe coating tube or sleeve 26, and an upper insert 34 that defines acentral exit passageway 36. The insert 34 has a frustoconical bottomsurface and several passageways in its thickened wall. The heatedlevitating and coating gases leaving the fluidized bed coater travelthrough this exit passageway 36 and through a suitable conduit 38leading to an appropriate vent.

A particle feeder 40 is mounted generally above the fluidized bed coater20 and is designed to feed minute particles 41 into the coatingenclosure at a desired rate. The particles from the feeder 40 enter thecoater through an entrance conduit provided by one of the passageways 42in the wall of the insert. A suitable induction or alternating currentheating device 44 is located in surrounding relationship to the furnacetube; it heats the active deposition region of the coating enclosure aswell as the small particles and the objects being levitated to bringthem to the desired temperature at which pyrolytic deposition occurs.

A coating operation is usually carried out by first establishing alevitated bed of minute particles of submillimeter size. This fluidizedbed is maintained in a lower coating region 47 of the coating chamber asillustrated. Separate objects to be coated within the particles thatcomprise the bed, for example annular heart valve bodies 45 ororthopedic implants, machined from dense isotropic graphite or the like,are appropriately loaded into the bed through the upper exit passageway36; they are then supported among the fluidized particles in the bedbeing levitated by the upwardly flowing gaseous stream. The temperaturewithin the coating enclosure is appropriately monitored and controlledto heat the particles and the substrates to the desired temperature.

When the coating process is ready to begin, a carbonaceous substance,such as a gaseous hydrocarbon (e.g., propane, propylene, methane, etc.or a mixture thereof) is added to the fluidizing gas stream. The supplyof the gaseous mixture is handled through flow-regulating valvearrangements that are part of a gas supply system 50 which operates toappropriately mix the hydrocarbon and the fluidizing gas. In theillustrated arrangement, the gas being supplied flows upward through thecentral passageway 32 and creates a generally toroidal flow pattern ofparticles within the coating enclosure; generally the particles andsubstrates flow upward in a central region of the bed and fan out at thetop of the bed, returning along the interior cylindrical sidewall of thetube 26.

The primary object of such coating operation will usually be to coat theexterior surfaces of small substrates tumbling in the fluidized bed ofsupporting particles, with a uniform coating of pyrocarbon. Thecharacter of the bed may be constantly changing because the minuteparticles in the bed grow in size as a result of the coating that isoccurring. For coating with pyrocarbon, an amount of small particles(e.g., spheroids) may be provided having sufficient total surface sothat there will be, for example, at least about 70 sq. cm of surface foreach cu. cm of volume in the active coating region. In an embodiment,such ancillary particles are about 1,500 microns or less in size with anaverage size not greater than about 800 microns. In order to maintain adesired, fairly constant bed character, a small amount of particles aregenerally continuously removed from the bed, and new particles (whichmay be substantially smaller in size) may be continuously added.Particle removal may be effected through an exit conduit 56 having anexit hole 57 in its side wall, and particles that enter this conduitfall by gravity through a passageway 56 a in the nozzle block 28. Theyultimately pass through a hole in the support plate 25 and enter aparticle removal system 63 where they are collected and weighed. Aspreviously indicated, they may be replaced with particles 41 of smallersize that are supplied by the particle feeder 40 and enter an upperregion of the fluidized bed.

In the illustrated arrangement, the size of the fluidized bed within thecoating enclosure may be regulated in order to precisely control thecrystalline character of the pyrocarbon being deposited. Control may beeffected by measuring the pressure differential across the fluidized bedby monitoring the pressure difference between a point at a lower regionin the bed or below the bed and a point above the bed. To this end, anupper pressure-sensing port 70 is provided in the upper insert 34, and alower pressure-sensing port 72 is provided at the end of a longpassageway in the nozzle block 28. The upper port 70 and the lower port72 may be respectively connected via tubing 74, 76 to a pressuretransducer 78 for measuring the pressure at these ports and thencomparing the two pressures measured to determine the differencetherebetween. To keep the ports and tubing clear of dust, carbonaceousmaterial and the like, an appropriate slow purge flow of inert gas maybe maintained through both port systems. A signal from the pressuretransducer 78 may be sent via line 84 to control unit 86, whichinstigates appropriate adjustments by sending signals to the particleremoval system 63 and the particle feeder 40 through lines 88 and 90.The control unit 86 also controls the gas supply system 50, sendingsignals via a line 92.

Illustrated in FIG. 1 is an embodiment with an arrangement for coatingsmall substrates 111, which are useful as orthopedic implants having theshape of a generally hemispherical shell or head 113 and a stem 115 thatextend axially from a central region of the concave surface of the thinshell or head. Although the head 113 does not have the shape of acomplete hemisphere, for convenience of description, it is referred toas hemispherical, even though its surface is only a segment or sectionof a sphere. In an embodiment, grouping six of these substrates 111 withtheir spherical convex surfaces juxtaposed with one another and theirstems 115 disposed essentially in three planes that are eachperpendicular to one another, may yield a uniform coating of the entiresurfaces of the substrates 111 because the confined group of sixsubstrates tumbles randomly within a fluidized bed coating region 47,such as exemplified in FIG. 5.

Maintenance of the six substrates 111 in this desired regular spatialorientation one to another is accomplished through the use of aconfining cage or holder 117. In an embodiment, the cage 117 comprisessix rings 119 that are regularly located as a part of athree-dimensional array, interconnected with one another by a total oftwelve legs 121. The legs 121 each have an arcuate central portion andshort straight end portions with each terminus thereof of fixed to oneof the rings 119 at four equiangularly (i.e. 90 degrees) spaced apartlocations on the ring. The cage 117 is proportioned so that the sixsubstrates 111 are accommodated therewithin with the respective stem 115of each protruding through one of the rings 119. The inner diameter ofeach ring 119 is sized so as to be slightly greater than the exteriordiameter of the stem 115 at the respective location along thefrustoconical surface thereof where the ring will surround it in theassemblage.

The overall arrangement is such that there will be no part of the cagethat will be in constant contact with any point on the surface of theimplant 111 being coated. In an embodiment, this arrangement of sixregularly spatially oriented substrates will generally continuouslytumble randomly within the fluidized bed of particles so there will beuniform exposure of the entire surfaces of the substrates to theupflowing stream of gas. In an embodiment, there will be slight movementof the six substrates 111 within the cage 117, but such movement will belimited. Axial movement of any one substrate in an inward direction willbe halted by contact with, for example, one or more of the fourspherical shells 113 that it faces, while the surrounding ring 119loosely confines the frustoconical stem. Movement of the substrate 113in the opposite axial direction would be met by its flat edge 123contacting one or more of the four legs 121 at one or more of fourpoints. The substrates 111 are free to rotate about their axes as therings 119 are sized so as not to retard relative rotation of the stem115 about its axis at any time. Such relative rotation will occur at thepoints where a substrate may contact the cage or another substrate. Inthis respect, the inner diameter of the ring 119 is designed in anembodiment so as to be larger by about 20-35%, or between about 25% andabout 30% larger than the diameter of the location on the frustoconicalstem 115 with which it will be aligned when the flat edge of thesubstrate 123 abuts the four confining legs 121.

Illustrated in FIG. 2 is an embodiment of a holder or cage 127 designedto confine six of the stemmed substrates 111 described above. The cage127 also includes six rings 129 which are equidistantly located in threemutually perpendicular planes by twelve legs 131. Unlike holder 117,with holder 127 legs 131 are sections of a circle or ring having aconstant radius of curvature. The result is a slightly morespherical-looking cage 127 as compared to the cage 117. As with the cage117, cage 127 is proportioned so that each of the six substrates canshift and relatively rotate about its axis within the cage but cannotexit from the cage or assume a position where two or more of thesubstrates would bind against the cage and one another.

Cages 117, 127 can be fabricated from any suitable materials that willwithstand the temperatures at which the vapor deposition processes willbe carried out. With respect to pyrocarbon coating processes,temperatures are used within the range of about 1200° C. to about 2000°C. For example, when a mixture of 40% propane and 60% nitrogen is used,the operating temperature may be maintained at about 1400° C. As anexample, a flow rate of about 20 liters per minute may be fed through acoater wherein the coating region has an interior diameter of about 3.5inches. Molybdenum and tantalum alloys are examples of suitablematerials for the fabrication of the cages. Although the cages may beformed of wire material of circular cross section, as illustrated in thedrawings, they could be fabricated from various types of perforated, cutand/or bent sheet metal; alternatively, they could be cast or suitablymachined.

A variety of arrangements may be used to facilitate the loading of acage with the plurality of substrates (e.g., six stemmed implants).Illustrated in FIG. 2 is one arrangement that might be employed; insteadof fastening four of the legs 131 a directly to the rings 129 a, asleeve connector 133 is affixed to the ring 129 a that will securelyreceive and hold the terminus of a leg 131 a. Accordingly, with thistype of arrangement, five of the substrates 111 can be loaded into thecage or holder 127, and then the sixth substrate with its stem 115extending through the ring 129 a can be mated to this by inserting therespective ends of the four legs 131 a into the socket-type sleeves 133.Although not illustrated, a similar arrangement would work as well otherembodiments such as cage 117.

When cage 127 with its six stemmed implants 111 is loaded into thecoating furnace through the upper end, it becomes submerged in thefluidized bed and is levitated along with the particles that form thebed by the upflowing gaseous stream. The six substrates have theirconvex spherical surfaces in a regular spatial orientation facing oneanother, and their axes are all aimed generally at the center of thesubstantially spherical enclosure provided by the cage 127. As a result,the assemblage will tumble generally continuously and randomly while inthe bed as the coating deposition progresses. The result is that all orsubstantially all of the surfaces of the stemmed substrates 111 aresubstantially equally exposed to the upflowing stream of gas, and arecoated generally uniformly across their entire exposed surfaces.

Illustrated in FIGS. 3 and 4 is another embodiment of a holder or a cage139 that is designed to support six substrates 141 in a regular spatialorientation with respect to one another for coating in a fluidized bedvia vapor decomposition. Substrates 141 are generally hemisphericalhollow shells, each having a convex spherical surface 143 and a concavegenerally spherical surface 145 that extends outwardly from a smallcircular top wall 147.

Holder 139 comprises a surrounding framework which includes a pluralityof straight sections 149 that lie generally along the surface of animaginary cube. They are interconnected with one another at their endsby arcuate bends or elbows 151 so they lie at 90 degree angles to oneanother. This cubic framework supports six legs 155 which extendradially inward so that the six of them all point toward the center ofthe cubic framework. The legs 155 are proportioned in length so thatthey will each support one generally hemispherical shell substrate 141at the desired interior location as shown in FIGS. 3 and 4.

Supporting interconnection between the tip of each leg 155 and the flatcircular top wall section 147 of the substrate may be effected in anysuitable manner desired. For example, substrate 141 could be providedwith a short blind hole centrally of the flat top wall surface 147 intowhich the tip of the leg could be press fit. Alternatively, the tip ofeach leg 155 could be threaded, and a short hole with mating threadscould be provided centrally of each circular flat top wall 147. Asanother alternative, the end of the leg might simply have a flat surfaceupon which a layer of adhesive could be provided that would besatisfactory to attach to the graphite substrate being coated andwithstand the high temperatures of the pyrolytic deposition.

It should be apparent from FIGS. 3 and 4 that the six substrates can beserially attached to the six legs 155 so that they are spatiallyregularly oriented with their convex spherical surfaces pointingspherically toward one another. When such an assemblage, as shown inFIGS. 3 and 4, of six substrates 141 supported within the holder 139, isloaded into the coating furnace throughout the open upper end, it willagain be submerged within the fluidized bed and will tumble continuouslyand randomly as it is levitated by the upflowing gas stream. As aresult, the entire surfaces of each generally hemispherical shellsubstrate will be uniformly coated with pyrocarbon or with whatevercoating is being deposited, to provide a uniform thickness. The onlysurface location that will not be coated would of course be the smallarea of contact between the leg and the top wall. However, if forexample the substrate is an orthopedic implant, this location would notbe an articular surface and thus less critical. Accordingly, the hole orother location within or upon what would likely be an isotropic graphitesubstrate might either be left as is or simply filled with a suitablebiocompatible material.

FIGS. 6A and 6B are side and perspective views of an embodiment of theinvention. The figures are not meant to be assembly drawings and aremerely illustrative of a general assemblage for providing even coatings(e.g., pyrocarbon) to substrates, such as biologic implants. Holder orcage 760 supports six substrates 750 in a regular spatial orientationwith respect to one another for coating in a fluidized bed via vapordecomposition. Other embodiments may include more or less than sixsubstrates while still providing regular orientation between substrates,which will avoid or lessen plum-bob orientations (and the consequentuneven coating).

In embodiments of FIGS. 6A-B, substrates 750 are generally hemisphericalhollow shells, each having a spherical surface 752 that extendsoutwardly to stem 751. Holder 760 comprises a surrounding frameworkwhich includes main supports 740 (shaded in FIG. 6B), base 720, crossrods 710, circular piece 731, and tie rods 700 (see FIGS. 7A-E). Holder760 may be made in numerous cage sizes to ensure differently sized partscan move inside cage enough to avoid constant contact with the cage(which can lead to the part fusing to the cage due to carbon coating)but not so lose as to inflict forces on the coating (due to excessiverattling) that might affect the structural integrity of the coating.

For example, the substrates are each confined such that spacing betweenany substrate 750 and any portion of cage 760 (e.g., as denoted by PointA of FIG. 6A) minimizes translation and/or rotation of the substratesduring the entirety of the deposition process without resulting inconstant contact for any of the substrates. Also, the substrates areeach confined such that spacing between any two substrates 750 (e.g., asdenoted by Point B of FIG. 6B) minimizes translation and/or rotation ofthe substrates during the entirety of the deposition process withoutresulting in constant contact for any of the substrates. In anembodiment, there is a maximum of 0.010 inches (e.g., during a collisionwhile tumbling) to 0.500 inches (e.g., while substrates are in the cagebut tumbling has yet to occur) of clearance between the substrate and anearest cage member or other substrate. Thus, cage 760 allows eachsubstrate to “jiggle” or move about during the random tumbling no morethan 0.010 to 0.500 inches in any direction. These ranges may ensureparts can move enough to avoid constant contact with the cage but not solose as to inflict forces on the coating that might affect thestructural integrity of the coating.

Cage 760 is a cage embodiment that includes four main supports 740, onebase 720, four cross rods 710, and twenty-four tie rods 700. Each cagemember may be, for example, made from 0.050″ diameter molybdenum.However, other embodiments are not so limited. Cage 760 is assembled byfirst using base 720 and four main supports 740 to assemble an outercube. Next, four cross rods 710 are respectively attached to oppositecorners of the cube, thereby extending across the middle portion (i.e.,central core) of the cube. Circular piece 730 is attached at theintersection of cross rods 710 to tie them together in the center of thecube. Afterwards, a single tie rod 700 is attached diagonally across oneface of the cube. Substrate 750 is then placed in the compartment withstem 751 pointing outwards from the cube (of course other orientationsare possible such as the stem pointing inwards). Another tie rod 700 isthen attached parallel to the first tie rod, also extending across thesame face of the cube. A third tie rod 700 is then attached 90 degreesto the first two tie rods and across the same face of the cube. Next afourth tie rod 700 is attached parallel to the third one and on the sameface of the cube. Now the stem is “boxed” in by four tie rods 700. Theabove steps are then repeated for the remaining five sides of the cubeto finally encompass six substrates. In an embodiment, the spacingbetween any substrate 750 and any tie rod can be adjusted by slightlypulling or bending the on the tie rod. The different members can beattached to one another by bending the ends about one another. When acage 760 is loaded into the coating furnace the cage will be submergedwithin the fluidized bed and will tumble substantially continuously andrandomly as it is levitated by the upflowing gas stream. As a result,generally all the surfaces of each implant will be substantiallyuniformly coated with pyrocarbon or with whatever coating is beingdeposited, to provide a uniform thickness.

Thus, an embodiment includes a method of coating substrates with auniform thickness of coating (and/or a cage for doing the same). Themethod includes (a) supplying a flow of a gaseous atmosphere (whichincludes a thermally decomposable vapor) to form a fluidized bed (whichmay or may not include particles in addition to substrates to becoated); (b) providing substrates (e.g., implants) with each substratehaving a similar asymmetrical shape (e.g., a shell with or without astem); (c) confining the substrates, in a container, in a generallystatic orientation with one another (i.e., where some substrate movementis allowed but in general the substrates maintain a consistentorientation with one another such as would be the case with, forexample, FIGS. 6A-B). The asymmetrical shape is one that would plum-bobif it were not confined in the container. The substrates would becollectively positioned evenly about a central core of the container.The central core may be defined as a spatially central portion of thecontainer, a center of gravity for the container, and the like. Forexample, two adjacent substrates may not be positioned symmetricallyabout a core of the container but two opposite substrates would be. By“collectively” the entirety of the substrates in the container would besymmetrically and evenly spaced and oriented about the core so as toprovide random tumbling and avoid plum-bobbing. The method may theninclude randomly tumbling the container of substrates within thefluidized bed based on the symmetrical orientation of the substrateswithin the container. The container may be configured to allow eachsubstrate to: (i) pivot about an axis (e.g., long axis extending alongstem 751) of the substrate, and (ii) jiggle (i.e., move about) duringthe random tumbling so the substrate is not in constant contact with anyother substrate or any portion of the container (to avoid fusing thesubstrate to another substrate or the container with the coating). Therandomly tumbling will then help provide a coating of generally uniformthickness on generally all portions of each substrate. Of course eachsubstrate may have geometrical conditions that preclude absolute evencoating (e.g., a deep channel that is difficult for vapor to penetrate)but such uneven coating would not be due to a plum-bob effect. Themethod may have two substrates oppositely oriented about a line ofsymmetry to each other. For example, such a situation is met by twosubstrates in FIG. 6A whose stems are parallel to each other butpointing in opposite directions. In an embodiment, the substrates eachinclude a shell portion 752 having convex and concave surfaces.

Again, cage 760 may be made in a variety of sizes. In variousembodiments, various ranges of dimensions (inches) are as follows: 701(0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4); 702 (2.25, 2.50, 2.75, 3.00,3.25, 3.50), 703 (0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16); 711(3.5, 4.0, 4.5, 5.0, 5.5, 6.0); 713 (0.09, 0.10, 0.11, 0.12, 0.13, 0.14,0.15, 0.16); 712 (0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18); 741and 742 (2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9); 743 (0.09,0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16); 744 (0.11, 0.12, 0.13, 0.14,0.15, 0.16, 0.17, 0.18); 721 and 723 (0.09, 0.10, 0.11, 0.12, 0.13,0.14, 0.15, 0.16); 724 (0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18);722 (2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9); 732 (0.15, 0.17,0.19, 0.21, 0.23, 0.25); and 731 (0.3, 0.4, 0.5, 0.6, 0.7).

Although various embodiments of the invention have been described, itshould be understood that various changes and modifications as would beobvious to one having ordinary skill in this art may be made withoutdeparting from the scope of the invention. For example, although thesubstrates which comprise generally hemispherical hollow shells havebeen shown (e.g., which are suitable for prospective orthopedic implantssuch as shoulder implants), other types of substrates (e.g., fingerimplants, hip implants, heart valves, other cardiovascular implants, andthe like) that might not individually tumble regularly within afluidized bed coater can be supported in a regular spatial orientationone to another in a cage or by a holder in order to provide anassemblage that will tumble continuously and randomly and thus assure agenerally uniform coating across the exterior surfaces thereof. Althoughfluidized pyrocarbon coating methods are described and illustrated,other chemical vapor deposition methods (e.g., using precursor feed gasdiluted in a carrier or diluant that is flowed upwardly to contact alevitated and heated substrate) may be employed to deposit coatings(e.g., Ti, Ta, W, Si from suitable halides, hydrides and the like) onsubstrates. It is intended that the appended claims cover all suchmodifications and variations as fall within the true spirit and scope ofthis present invention.

What is claimed is:
 1. A system to coat substrates with a coating ofuniform thickness comprising: substrates of similar asymmetrical shapeto be coated, and a container confining the substrates in a generallystatic orientation with one another such that the substrates arecollectively positioned evenly about a central core of the container;wherein the container is configured to allow each substrate to: (a)randomly tumble in a flowing gaseous stream based on the generallystatic orientation of the substrates in the container; (b) pivot aboutan axis of the respective substrate, (c) jiggle during the randomtumbling so the respective substrate is not in constant contact with anyother substrate or any portion of the container; and (d) receive acoating of generally uniform thickness on generally all portions of therespective substrate based on the random tumbling.
 2. The system ofclaim 1 wherein the substrates comprise at least two substrates.
 3. Thesystem of claim 2 wherein the substrates are confined with a regularorientation to one another so as to provide a view that is substantiallythe same when viewed in any one of three perpendicular planes.
 4. Thesystem of claim 2 wherein each substrate is configured to plum-bobwithin the fluidized bed, if not confined in the container, based on theasymmetrical shape of the respective substrate.
 5. The system of claim 2wherein the container includes a fixture to which each of the substratesis attached at a single location to secure them in rigid orientation oneto another.
 6. The system of claim 5 wherein each substrate isconfigured to plum-bob within the fluidized bed, if not confined in thecontainer, based on the asymmetrical shape of the respective substrate.7. A system to coat substrates with a coating of uniform thicknesscomprising: a container configured to confine substrates of similarasymmetrical shape in a generally static orientation with one anothersuch that the substrates are collectively positioned evenly about acentral core of the container; wherein the container is configured toallow each substrate to: (a) randomly tumble in a flowing gaseous streambased on the generally static orientation of the substrates in thecontainer; (b) pivot about an axis of the respective substrate, (c)jiggle during the random tumbling so the respective substrate is not inconstant contact with any other substrate or any portion of thecontainer; and (d) receive a coating, based on materials included in thegaseous stream, of generally uniform thickness on generally all portionsof the respective substrate based on the random tumbling.
 8. The systemof claim 7 wherein the substrates comprise at least two substrates. 9.The system of claim 8 wherein the container is configured to confine thesubstrates with a regular orientation to one another so as to provide aview that is substantially the same when viewed in any one of threeperpendicular planes.
 10. The system of claim 8 wherein the containerbeing configured to allow each substrate to jiggle includes thecontainer being configured to allow each substrate to move back andforth along multiple axes of movement during the random tumbling. 11.The system of claim 8 wherein the container includes a fixture to whicheach of the substrates is attached at a single location to secure themin rigid orientation one to another.
 12. The system of claim 7 whereineach substrate is configured to plum-bob within the fluidized bed, ifnot confined in the container, based on the asymmetrical shape of therespective substrate.
 13. The system of claim 12 wherein each of thesubstrates includes a generally hemispheric shell and a stem extendingfrom the shell.
 14. The system of claim 13 wherein the containerincludes rings sized to loosely receive the stem extending from theshell of each of the substrates.
 15. The system of claim 14, whereineach of the rings is monolithic and circular.
 16. The system of claim 7wherein the coating includes pyrocarbon.
 17. The system of claim 7,wherein the container is configured to allow each substrate to slideaxially both towards a center of the container and away from the centerof the container during the random tumbling.